Sodium carbonate (Na2CO3), or soda ash, is one of the largest volume alkali commodities made worldwide with a total production in 2008 of 48 million tons. Sodium carbonate finds major use in the glass, chemicals, detergents, paper industries, and also in the sodium bicarbonate production industry. The main processes for sodium carbonate production are the Solvay ammonia synthetic process, the ammonium chloride process, and the trona-based processes.
Trona-based soda ash is obtained from trona ore deposits in the U.S. (southwestern Wyoming in Green River, in California near Searles Lake and Owens Lake), Turkey, China, and Kenya (at Lake Magadi) by underground mechanical mining techniques, by solution mining, or lake waters processing.
Crude trona is a mineral that may contain up to 99% sodium sesquicarbonate (generally about 70-99%). Sodium sesquicarbonate is a sodium carbonate—sodium bicarbonate double salt having the formula (Na2CO3.NaHCO3.2H2O) and which contains 46.90 wt. % Na2CO3, 37.17 wt. % NaHCO3 and 15.93 wt. % H2O. Crude trona also contains, in lesser amounts, sodium chloride (NaCl), sodium sulfate (Na2SO4), organic matter, and insolubles such as clay and shales. A typical analysis of the trona ore mined in Green River is shown in TABLE 1.
Other naturally-occurring sodium (bi)carbonate minerals from which sodium carbonate and/or sodium bicarbonate may be produced are known as nahcolite, a mineral which contains mainly sodium bicarbonate and is essentially free of sodium carbonate and known as “wegscheiderite” (also called “decemite”) of formula:Na2CO3.3NaHCO3.
TABLE 1ConstituentWeight PercentNa2CO343.2-45  NaHCO333.7-36  H2O (crystalline and free moisture)15.3-15.6NaCl0.004-0.1 Na2SO40.005-0.01 Insolubles3.6-7.3
In the United States, trona and nahcolite are the principle source minerals for the sodium bicarbonate industry. While sodium bicarbonate can be produced by water dissolution and carbonation of mechanically mined trona ore or of soda ash produced from trona ore, sodium bicarbonate can be produced also by solution mining of nahcolite. The production of sodium bicarbonate typically includes cooling crystallization or a combination of cooling and evaporative crystallization.
The large deposits of mineral trona in the Green River Basin in southwestern Wyoming have been mechanically mined since the late 1940's and have been exploited by five separate mining operations over the intervening period. In 2007, trona-based sodium carbonate from Wyoming comprised about 90% of the total U.S. soda ash production. To recover valuable alkali products, the so-called ‘monohydrate’ commercial process is frequently used to produce soda ash from trona. When the trona is mechanically mined, crushed trona ore is calcined (i.e., heated) to convert sodium bicarbonate into sodium carbonate, drive off water of crystallization and form crude soda ash. The crude soda ash is then dissolved in water and the insoluble material is separated from the resulting solution. A clear solution of sodium carbonate is fed to a monohydrate crystallizer, e.g., a high temperature evaporator system generally having one or more effects (sometimes called ‘evaporator-crystallizer’), where some of the water is evaporated and some of the sodium carbonate forms into sodium carbonate monohydrate crystals (Na2CO3.H2O). The sodium carbonate monohydrate crystals are removed from the mother liquor and then dried to convert the crystals to dense soda ash. Most of the mother liquor is recycled back to the evaporator system for additional processing into sodium carbonate monohydrate crystals.
The Wyoming trona deposits are evaporites and hence form various substantially horizontal layers (or beds). The major deposits consists of 25 near horizontal beds varying from 4 feet (1.2 m) to about 36 feet (11 m) in thickness and separated by layers of shales. Depths range from 400 ft (120 m) to 3,300 ft (1,000 m). These deposits contain from about 88% to 95% sesquicarbonate, with the impurities being mainly dolomite and calcite-rich shales and shortite. Some regions of the basin contain soluble impurities, most notably halite (NaCl). These extend for about 1,000 square miles (about 2,600 km2), and it is estimated that they contain over 75 billions tons of soda ash equivalent, thus providing reserves adequate for reasonably foreseeable future needs.
In particular, a main trona bed (No. 17) in the Green River Basin, averaging a thickness of about 8 feet (2.4 m) to about 11 feet (3.3 m) is located from approximately 1,200 feet (about 365 m) to approximately 1,600 feet (about 488 m) below ground surface. Presently, trona from the Wyoming deposits is economically recovered mainly from the main trona bed no. 17. This main bed is located below substantially horizontal layers of sandstones, siltstones and mainly unconsolidated shales. In particular, within about 400 feet (about 122 m) above the main trona bed are layers of mainly weak, laminated green-grey shales and oil shale, interbedded with bands of trona from about 4 feet (about 1.2 m) to about 5 feet thick (about 1.5 m). Immediately below the main trona bed lie substantially horizontal layers of somewhat plastic oil shale, also interbedded with bands of trona. Both overlying and underlying shale layers contain methane gas.
The comparative tensile strengths, in pounds per square inch (psi) or kilopascals (kPa), of trona and shale in average values are substantially as follows:                Shale: 70-140 psi (482-965 kPa)        Trona: 290-560 psi (2,000-3,861 kPa)        
Both the immediately overlying shale layer and the immediately underlying shale layer are substantially weaker than the main trona bed. Recovery of the main trona bed, accordingly, essentially comprises removing the only strong layer within its immediate vicinity.
Most mechanical mining operations to extract trona ore practice some form of underground ore extraction using techniques adapted from the coal and potash mining industries. A variety of different systems and mechanical mining techniques (such as longwall mining, shortwall mining, room-and-pillar mining, or various combinations) exist. Although any of these various mining techniques may be employed to mine trona ore, when a mechanical mining technique is used, nowadays it is preferably longwall mining.
All mechanical mining techniques require miners and heavy machinery to be underground to dig out and convey the ore to the surface, including sinking shafts of about 800-2,000 feet (about 240-610 meters) in depth. The cost of the mechanical mining methods for trona is high, representing as much as 40 percent of the production costs for soda ash. Furthermore, recovering trona by these methods becomes more difficult as the thickest beds (more readily accessible reserves) of trona deposits with a high quality (less contaminants) were exploited first and are now being depleted. Thus the production of sodium carbonate using the combination of mechanical mining techniques followed by the monohydrate process is becoming more expensive, as the higher quality trona deposits become depleted and labor and energy costs increase. Furthermore, development of new reserves is expensive, requiring a capital investment of as much as hundreds of million dollars to sink new mining shafts and to install related mining and safety (ventilation) equipment.
Additionally, because some shale is also removed during mechanical mining, this extracted shale must be transported along with the trona ore to the surface refinery, removed from the product stream, and transported back into the mine, or a surface waste pond. These insoluble contaminants not only cost a great deal of money to mine, remove, and handle, they provide very little value back to the mine and refinery operator. Additionally, the crude trona is normally purified to remove or reduce impurities, primarily shale and other nonsoluble materials, before its valuable sodium content can be sold commercially as: soda ash (Na2CO3), sodium bicarbonate (NaHCO3), caustic soda (NaOH), sodium sesquicarbonate (Na2CO3.NaHCO3.2H2O), a sodium phosphate (Na5P3O10) or other sodium-containing chemicals.
Recognizing the economic and physical limitations of underground mechanical mining techniques, solution mining of trona has been long touted as an attractive alternative with the first patent U.S. Pat. No. 2,388,009 entitled “Solution Mining of Trona” issued to Pike in 1945. Pike discloses a method of producing soda ash from underground trona deposits in Wyoming by injecting a heated brine containing substantially more carbonate than bicarbonate which is unsaturated with respect to the trona, withdrawing the solution from the formation, removing organic matter from the solution with an adsorbent, separating the solution from the adsorbent, crystallizing, and recovering sodium sesquicarbonate from the solution, calcining the sesquicarbonate to produce soda ash, and re-injecting the mother liquor from the crystallizing step into the formation.
In its simplest form, solution mining of trona is carried out by contacting trona ore with a solvent such as water or an aqueous solution to dissolve the ore and form a liquor (also termed ‘brine’) containing dissolved sodium values. For contact, the water or aqueous solution is injected into a cavity of the underground formation, to allow the solution to dissolve as much water-soluble trona ore as possible, and then the resulting brine is extracted to the surface. A portion of the brine can be used as feedstock to one or more processes to manufacture one or more sodium-based products, while another brine portion may be re-injected for additional contact with trona.
Solution mining of trona could indeed reduce or eliminate the costs of underground mining including sinking costly mining shafts and employing miners, hoisting, crushing, calcining, dissolving, clarification, solid/liquid/vapor waste handling and environmental compliance. The numerous salt (NaCl) solution mines operating throughout the world exemplify solution mining's potential low cost and environmental impact. But ores containing sodium carbonate and sodium bicarbonate (trona, wegscheiderite) have relatively low solubility in water at room temperature when compared with other evaporite minerals, such as halite (mostly sodium chloride) and sylvite (mostly potassium chloride), which are mined “in situ” with solution mining techniques.
Implementing a solution mining technique to exploit sodium (bi)carbonate-containing ores like trona ore, especially those ores whose thin beds, beds of lower trona quality (e.g., less than 70% quality), and/or deep beds of depth greater than 2,000 ft (610 m) which are currently not economically viable via mechanical mining techniques, has proven to be quite challenging.
In 1945, Pike proposed the use of a single well comprising an outer casing and an inner casing. Hot solvent is injected through the inner casing to contact the trona bed, and the brine is withdrawn through the annulus. This method however proved unsuccessful, and currently there are two approaches to trona solution mining that are being pursued.
A hybrid approach to trona solution mining which is commercially used at the present time is part of an underground tailings disposal projects. Mine operators flood old workings, dissolving the pillars and recovering the dissolved sodium value. Solution mining of mine pillars was disclosed in U.S. Pat. No. 2,625,384 issued to Pike et al in 1953 entitled “Mining Operation”; it uses water as a solvent under ambient temperatures to extract trona from existing mined sections of the trona deposits. Solvay Chemicals, Inc. (SCI), known then as Tenneco Minerals was the first to begin depositing tails, from the refining process back into these mechanically mined voids left behind during normal partial extract operation. This hybrid approach takes advantage of the remnant voids and subsequent exposed surface areas of trona left behind from mechanical mining to both deposit insoluble materials and other contaminants (collectively called tailings or tails) and to recover sodium value from the aqueous solutions used to carry the tails.
Even though ‘hybrid’ solution mining is one of the preferred mining methods in terms of both safety and productivity, this method is necessarily dependent upon the surface area and openings provided by mechanical mining to make them economically feasible and productive, and there is a finite amount of trona that has been previously mechanically mined. The ‘hybrid’ solution mining cannot exist in their present form without the necessity of prior mechanical mining in a partial production mode. When current trona target beds will be completely mechanically mined, the mine operators will eventually be forced to move into thinner beds and/or into beds of lower quality and to endure more rigorous mining conditions while the more desirable beds are depleting and finally become exhausted.
A more sustainable approach to trona solution mining would allow the extraction from less desirable beds (thin beds, poor quality beds, and/or deeper beds) which are currently less economically viable, without the negative impact of increased mining hazards and increased costs. In such approach, two or more wells are drilled into the trona bed, and fluid communication between the wells is established by hydraulic fracturing or directional drilling.
Attempts to solution mine trona using vertical boreholes began soon after the 1940's discovery of trona in the Green River Basin in Wyoming. U.S. Pat. No. 3,050,290 entitled “Method of Recovery Sodium Values by Solution Mining of Trona” by Caldwell et al. discloses a process for solution mining of trona that suggests using a mining solution at a temperature of the order of 100-200° C. This process requires the use of recirculating a substantial portion of the mining solution removed from the formation back through the formation to maintain high temperatures of the solution. A bleed stream from the recirculated mining solution is conducted to a recovery process during each cycle and replaced by water or dilute mother liquor. U.S. Pat. No. 3,119,655 entitled “Evaporative Process for Producing Soda Ash from Trona” by Frint et al discloses a process for the recovery of soda ash from trona and recognizes that trona can be recovered by solution mining. This process includes introduction of water heated to about 130° C., and recovery of a solution from the underground formation at 90° C.
Directional drilling from the ground surface has been used to connect dual wells for solution mining bedded evaporite deposits and the production of sodium bicarbonate, potash, and salt. Nahcolite solution mining utilizes directionally drilled boreholes and a hot aqueous solution comprised of dissolved soda ash, sodium bicarbonate, and salt. Development of nahcolite solution mining cavities by using directionally drilled horizontal holes and vertical wells is described in U.S. Pat. No. 4,815,790, issued in 1989 to E. C. Rosar and R. Day, entitled “Nahcolite Solution Mining Process”. The use of directional drilling for trona solution mining is described in U.S. Patent Application Pre-Grant Publication No. US 2003/0029617 entitled “Application, Method and System For Single Well Solution Mining” by N. Brown and K. Nesselrode. A well pair per cavity may be used for injection and production. A plurality of lateral boreholes in various configurations such as those described in U.S. Pat. No. 8,057,765, issued in November 2011 to Day et al, entitled “Methods for Constructing Underground Borehole Configurations and Related Solution Mining Methods” is described to improve the lateral expansion of a solution mined cavity in the evaporite deposit.
In the late 1950's-early 1960's, hydraulic fracturing of trona has been proposed, claimed or discussed in patents as a means to connect two wells positioned in a trona bed. See for example U.S. Pat. No. 2,847,202 (1958) by Pullen, entitled “Methods for Mining Salt Using Two Wells Connected by Fluid Fracturing”; U.S. Pat. No. 2,952,449 (1960) by Bays, entitled “Method of Forming Underground Communication Between Boreholes”; U.S. Pat. No. 2,919,909 (1960) by Rule entitled “Controlled Caving For Solution Mining Methods”; U.S. Pat. No. 3,018,095 (1962) by Redlinger et al, entitled “Method of Hydraulic Fracturing in Underground Formations”; and GB 897566 (1962) by Bays entitled “Improvements in or relating to the Hydraulic Mining of Underground Mineral Deposits”. 
In the 1980's, a borehole trona solution mine attempt by FMC Corporation involved connecting multiple conventionally drilled vertical wells along the base of a preferred trona bed by the use of hydraulic fracturing. FMC published a report (Frint, Engineering & Mining Journal, September 1985 “FMC's Newest Goal: Commercial Solution Mining Of Trona” including “Past attempts and failures”) promoting the hydraulic fracture well connection of well pairs as the new development that would commercialize trona solution mining. According to FMC's 1985 article though, the application of hydraulic fracturing for trona solution mining was found to be unreliable. Fracture communication attempts failed in some cases and in other cases gained communication between pre-drilled wells but not in the desired manner. The fracture communication project was eventually abandoned in the early 1990's.
These attempts of in situ solution mining of virgin trona in Wyoming were met with less than limited success, and technologies using hydraulic fracturing to connect wells in a trona bed failed to mature.
In the field of oil and gas drilling and operation however, hydraulic fracturing is a mainstay operation, and it is estimated that more than 60% new wells in 2011 used hydraulic fracturing to extract shale gas. Such hydraulic fracturing often employs directional drilling with horizontal section within a shale formation for the purpose of opening up the formation and increasing the flow of gas therefrom to a particular single well using multi-fracking events from one horizontal borehole in the formation.
Through this technique, it has been established that fractures produced in formations should be approximately perpendicular to the axis of the least stress and that in the general state of stress underground, the three principal stresses are unequal (anisotropic conditions). Where the main stress on the formation is the stress of the overburden, these fractures tend to develop in a vertical or inverted conical direction. Horizontal fractures cannot be produced by hydraulic pressures less than the total pressure of the overburden.
In fracturing between spaced wells in evaporite mineral formations for the purpose of removing the mineral by solution flowing between the adjacent wells, the ‘fracking’ methods used in the oil & gas industry are however not suitable to accomplish the formation of a single main horizontal fracture. Because the depth of the hydraulically-fractured formation is generally greater than 1,000 meters (3,280 ft), the injection pressures in oil & gas exploration are high, even though they are still less than the overburden pressure; this favors the formation of vertical fractures which increases permeability of the exploited shale formation. The main goal of ‘fracking’ methods in the oil & gas industry is indeed to increase the permeability of shale. Overburden gradient is generally estimated to be between 0.75 psi/ft (17 kPa/m) and 1.05 psi/ft (23.8 kPa/m), thus what is called the ‘fracture gradient’ used in oil & gas fracking is less than the overburden gradient, preferably less than 1 psi/ft (22.6 kPa/m), preferably less than 0.95 psi/ft (21.5 kPa/m), sometimes less than 0.9 psi/ft (20.4 kPa/m). The ‘fracture gradient’ is a factor used to determine formation fracturing pressure as a function of well depth in units of psi/ft. For example, a fracture gradient of 0.7 psi/ft (15.8 kPa/m) in a well with a vertical depth of 2,440 m (8,000 ft) would provide a fracturing pressure of 5,600 psi (38.6 MPa).
Unlike the oil and gas exploration from shale formations where it is desirable to produce numerous vertical fractures near the center of the shale formation to recover the most oil and/or gas therefrom, in the recovery of a soluble mineral from underground evaporite formations, it is desirable to produce a single fracture substantially at the bottom of the evaporite mineral stratum and along the top of the underlying water-insoluble non-evaporite stratum and to direct the fracture to the next adjacent well along the interface between the bottom of the evaporite stratum to be removed and the top of the underlying stratum so that the soluble mineral will be dissolved from the bottom up.
The bottom-up approach for dissolving the mineral from the interface gap (fracture) created substantially at the bottom of the evaporite stratum offers a number of advantages. The less concentrated and less saturated solvent present in the gap rises to a top layer of the solvent body inside the gap due to density gradient, and contacts the roof of the evaporite stratum cavity, dissolves the mineral therefrom, and as the solvent becomes more saturated, settles to a lower layer of the solvent body so that the bottom edge of the evaporite stratum is always exposed to dissolution by less concentrated solvent. The insoluble materials in the evaporite formation can settle through the solvent body to the bottom of the solution-mining cavity and deposit thereon so that only clear solutions are recovered from production wells.
A further advantage of the bottom-up approach for solution mining of mineral from a mature mineral cavity is that it can help minimize contact of the solvent with contaminants-rich minerals (e.g., halite) which may be found in overlying strata such as green shale strata found above a trona stratum. Since these contaminants-rich minerals are generally soluble in the same solvent as the desirable mineral, if solvent flow is allowed to occur to reach contaminated overlying layers, this would allow contaminants from these overlying layers to dissolve into the solvent, thereby “poisoning” the resulting brine and rendering it useless or, at the very least, making its further processing into valuable product(s) very expensive. Indeed, poisoning by sodium chloride from chloride-based minerals can occur during solution mining of trona, and it is suspected that the solution mining efforts by FMC in the 1980's in the Green River Basin were mothballed in the 1990's due to high NaCl contamination in the extracted brine.
Other than chloride poisoning, another complicating factor in dissolving in situ underground double-salt ores like an ore containing sodium sesquicarbonate (main component of trona) or wegscheiderite is that sodium carbonate and sodium bicarbonate have different solubilities and dissolving rates in water. These incongruent solubilities of sodium carbonate and sodium bicarbonate can cause sodium bicarbonate ‘blinding’ (also termed ‘bicarb blinding’) during solution mining. Blinding occurs as the bicarbonate, which has dissolved in the mining solution tends to redeposit out of the solution onto the exposed face of the ore as the carbonate saturation in the solution increases, thus clogging the dissolving face and “blinding” its carbonate values from further dissolution and recovery. Blinding can thus slow dissolution and may result in leaving behind significant amounts of reserves in the mine. It can be shown that the aforementioned problem arises because when trona, for example, is dissolved in water, both the sodium bicarbonate and the sodium carbonate fractions begin going into solution at the same time until the solution reaches saturation with respect to sodium bicarbonate. Unfortunately, the resulting liquid phase existing at this point is in equilibrium with sodium bicarbonate in solid phase, and the sodium carbonate continues to dissolve while the bicarbonate starts precipitating out until the final resulting solution is in equilibrium condition with sodium sesquicarbonate (trona) as the stable solid phase, in fact, reached wherein a substantial portion of sodium bicarbonate precipitates out of solution and a lot more of the sodium carbonate has gone into solution. Wegscheiderite behaves in much the same way as trona in that they both go into solution in accordance with their respective solid percentage compositions of sodium bicarbonate and sodium carbonate. It is expected that the deposited sodium bicarbonate is most likely prevalent around a downhole end of a production well during dissolution phase (a), when the sodium bicarbonate content in the brine surrounding the downhole end of this well may be saturated or super-saturated under the conditions of dissolution in this area of the cavity.
Additionally, a phenomenon termed ‘channeling’ in an ore bed may occur during solution mining. A ‘channeling’ event describes the tendency of the solvent to find and maintain a path through an area of ore insolubles (e.g., trona insolubles). Once a channel is created, it may result in low or near zero dissolution rates of the surrounding ore, as the solvent bypasses solute-containing ore and fails to expose the mineral solute to the solvent. It is expected however that this phenomenon may not occur or may be disrupted when the solvent flow path is modified periodically.
Some of the problems of prior art solution mining techniques, for example the formation of “morning glory” holes which are generally narrow at the base and flare outward at the top in a generally convex upward cross-sectional floor profile. A variety of techniques have been attempted in order to prevent the formation of such types of holes, since they are very wasteful and since they result in a low percentage of mineral recovery from the bed. One of these techniques involves use of a blanket fluid above the level of the solvent in the cavity to achieve a more or less cylindrical solution-mined cavity. The contact between the solvent and the roof of the ore is prevented by the blanket fluid which is less dense than the solvent (such as a liquid lighter than water, e.g., diesel or liquefied petroleum gas, or a gas, e.g., pressurized air, nitrogen). This blanket fluid forces contact of solvent with the cavity walls, thus controlling the expansion of the cavity in the horizontal direction. But because the blanket fluid prevents contact of solvent with a large surface area of mineral ore on the mineral cavity ceiling, the dissolution rate can be greatly reduced.
Based on the foregoing, there is still a need for a solution mining method which addresses at least one or more of the issues provided above.